In the German Pat. No. 22 40 216, there is described a digitally controlled variable frequency synthesizer using a phase-locked loop circuit for control of the electronically tunable oscillator, namely the voltage-control oscillator whose output is at a frequency fa controlled by a selection input FA applied to a logic and arithmetic circuit (ALU).
While that circuit is described in greater detail below, it can be pointed out that it basically comprises a frequency divider which receives an input fa from the electronically tunable or voltage-controlled oscillator and has its division ratio m controlled by the ALU.
The output from the frequency divider is applied to a phase detector or phase meter which also receives a reference frequency fr from a fixed-divisor frequency divider supplied by a crystal-controlled, fixed-value or reference oscillator. A crossover switch can, both for the invention and its earlier circuit, be used to apply the output of the fixed-value oscillator to the loop for calibration purposes.
The output of the phases detector is ultimately applied as a control voltage to the control-voltage input of the voltage controlled oscillator or VCO.
The ALU in this circuit allows control of the dividing ratio of the variable frequency divider so that output frequency fa of the VCO can be selected in response to the value of FA. Experience with this circuit has shown, however, that there is a tendency for the output voltage of the phase detector to fluctuate or oscillate in a sawtooth manner in a way which cannot be fully eliminated by feeding the output of the phase detector through a low-pass filter to the control input of the VCO.
As a consequence, the output of the VCO can vary in an undesirable manner, the frequency resolution of the circuit may be limited and the frequency level of the reference frequency source can be limited, thereby adversely affecting the range of the synthesizer output.